Time-of-flight SIMS is the standard mass spectrometric tool for the molecular analysis of biologically relevant surfaces, but variations in desorption yields, ionization effic iencies, and ion-induced damage of adsorbates often present difficulties in the analysis of SIMS data. Laser desorption combined with photoionization may provide certain advantages over TOF-SIMS, but little data has been collected which compares the relative efficacy of these methods. To make such a comparison, biotin and biotin-conjugated self-assembled monolayers are being used as model systems. Surface coverages are monitored independently by ESCA.